The Earth may be more than 4.5 billion years old, but it can still surprise us. Scientists have discovered a gigantic liquid water reservoir underneath Greenland’s massive ice sheet. (The discovery was reported in an article in this week’s edition of Nature Geoscience.) Uncovered accidentally by a team of glaciologists who were drilling ice cores in southeastern Greenland in 2011, the aquifer is more than 27,000 sq. miles (69,930 sq. km) large—bigger than West Virginia—according to data from NASA’s Operation Icebridge radar. And until recently, no one had any idea it was there.

That’s because, on the surface at least, it shouldn’t be there. Temperatures in Greenland are well below freezing most of the year, which is why much of the island is covered by a sheet of ice that is more than a mile (1.6 km) thick. Yet when the glaciologists drilling in 2011—led by the University of Utah’s Rick Forster—extracted their deep ice cores, they were surprised to find them dripping with liquid water, despite temperatures that were below 0º F (-18º C). Later research—carried out in April 2013 with Lora Koenig, a glaciologist with NASA’s Goddard Space Flight Center—found that the temperature in the aquifer stayed around 32º F (0º C), just above freezing. It’s possible that the heavy snow cover in southeastern Greenland may act as insulation for the aquifer—a literal blanket of snow—preventing it from freezing.

The volume of the aquifer—which is fed by meltwater that flows through the Greenland ice sheet—is immense, an estimated 154 billion tons of water. That would be enough by itself to raise global sea levels by 0.016 in (.04 cm) were the entire underground lake to flow into the oceans. That may not sound like a whole lot—the seas already rise by more than that amount each year, thanks to melting ice sheets and thermal expansion of warming seawater—but the discovery of the aquifer should help scientists better understand how melt water moves through the Greenland ice sheet.

That’s important because as the climate has warmed, the pace of ice loss in Greenland has accelerated, from 121 billion tons a year from 1993 to 2005 to 229 billion tons a year between 2005 and 2010. Better understanding of the physics governing the way the ice sheet, snow and meltwater interact could help scientists predict how Greenland will respond to warming in the future. And that matters because—never mind the fraction of an inch of sea level rise the newly discovered lake could cause—there’s enough frozen water locked in Greenland’s ice sheet to raise global sea levels by more than 20 ft. (6 m) were it all to melt. Scientists may like surprises when they’re discovering an underground lake, but when it comes to the threat of climate change, a little certainty would be preferable.